This kind of transmission is known from German Patent Application 36 05 203 A1. It enables a continuous step-free speed ratio adjustment in all of eight selectable speed ratio gears by using a mechanical power branching by means of a double planetary gear branched to a hydrostatic adjustment mechanism. In this transmission, the shaft on which the selectable ratio gears of the first through the fourth gears act comprises intermediate shafts, which can be connected in a controlled fashion with a transmission output shaft via group gears with associated group clutches. Acceleration is done here with a friction clutch, which is subject to wear and entails energy losses, located before both the mechanical and the hydrostatic gears. The gear shift clutches are also synchronized and thus suffer wear. The clutches of the 1st through 4th gears are disposed on the hollow shafts of the planetary gear, and as a result are relatively large. There is no provision for operation of the overall transmission in reverse, which severely limits the utilization of the transmission.
German Patent Application 38 15 780 A1 also discloses a hydrostatic-mechanical, split-type transmission with four-speed selectable ratio ranges, in which an input shaft is permanently operatively connected via a first spur gear to the variable-displacement gear, and furthermore, via a selectable reversing gear, can be coupled in controlled fashion with a planet shaft of a second of the planetary gears and the outer wheel of the first of the planetary gears, and an output shaft of the hydrostatic variable-displacement gear is connected via a toothed wheel gear to a sun wheel shaft on which the two sun wheels of the two planetary gears are disposed, and a planet shaft of the first planetary gear can be connected in controlled fashion to a shaft via first selectable ratio gears by associated clutches of a 1st and a 3rd selectable gear, and an outer wheel shaft of the second planetary gear can be coupled in controlled fashion to the named shaft via further selectable ratio gears by associated clutches of a 2nd and a 4th selectable gear. This transmission produces the same degree of conversion in the forward and reverse directions, but it is limited because it has only four speed ratio ranges and so can be used only for automobiles, but not for heavy-loaded farm machinery. Moreover, the reversing gear is selectively coupled by synchronizing clutches, which are subject to wear and the synchronization of which entails an energy loss.
A hydrostatic-mechanical coupling transmission with an input-side torque split is also known, from German Patent 31 47 447, which includes two three-shaft epicyclic gears I and II, of which a differential shaft of the first epicyclic gear I and the summation shaft of the second epicyclic gear II form a unit with the driving shaft, and of which coupling transmission the second differential shaft of the first epicyclic gear I is coupled to a differential shaft of the second epicyclic gear II and is connected to the driving shaft via a hydrostatic variable-displacement gear, and of which coupling transmission finally the summation shaft of the first epicyclic gear I, on the one hand, and the second differential shaft of the second epicyclic gear II, on the other hand, can act in alternative selection via higher-stage group gears, so that by variably running through the adjusting range of the hydrostatic gear, a plurality of stepless successive adjusting ranges form the total adjusting range of the transmission, the higher-stage group gears being adapted in their speed increasing ratios such that the adjusting range change at synchronized rotational speed takes place without load and without interruption of traction force on both sides of the clutches to be coupled or decoupled, but a friction clutch is provided for acceleration. The known transmission has four forward speed ranges and one reverse gear range and is thus suitable for passenger car operation; the total conversion ratio of the transmission amounts approximately to a factor of 8, which is usually not adequate for utility vehicles.
A four-speed ratio range transmission with hydrostatic power branching is also known from German Patent Disclosure Document DE-OS 24 23 626, in which an increase in efficiency can be attained by providing that at operating points in which the driven shaft of the hydraulic converter is practically at a standstill, it is stopped toward the housing by a clutch, so that only the mechanical gear is operative, and the converter is idling, virtually without loss. It is also provided that at operating points in which the input shaft and output shaft of the hydraulic converter are in synchronism, these shafts are firmly connected to one another by a further, separate clutch, so tat the converter rotates with them with low loss, and the power is transmitted purely mechanically.